The threat to the nuclear facility came as Gaza saw its bloodiest day of Israel’s Operation Protective Edge, with at least 22 people, reportedly including nine children and six women, killed in air strikes.

But the strikes failed to staunch the rocket fire from Hamas, which unleashed its full arsenal of long-range missiles as far as Tel Aviv and Jerusalem.

The exchanges were matched by bellicose political language on either side.

Mahmoud Abbas, the president of the Palestinian Authority, accused Israel of committing “genocide” in Gaza.

That claim was brushed aside by Shimon Peres, Israel’s president, who accused Hamas of starting the conflict by firing rockets. “We asked them to stop. We waited one day, two days, three days and they continued, and they spread their fire on more areas in Israel,” he said.

He added that a ground offensive on Gaza “may happen quite soon.” Referring to rockets being fired from Gaza, he added: “If they will stop for example tonight, there won’t be any ground entrance – but if they will continue, sooner or later this will be the response.”

Despite the rising number of dead, diplomatic pressure on Israel has so far been relatively light.

The European Union, through the office of its foreign policy chief, Baroness Ashton, condemned “indiscriminate” rocket fire from the Gaza Strip and deplored “the growing number of civilian casualties, reportedly among them children, caused by Israeli retaliatory fire.”

But the calls fell far short of the international chorus of criticism that accompanied Israel’s previous incursions into Gaza in 2008 and 2012. That, however, may be due to the relative absence of any single major tragedy so far to mobilise international opinion around – unlike in 2008, when the opening day of Operation Cast Lead saw several dozen Gazan police cadets killed by an Israeli drone that strafed their graduation ceremony.

“Presumably there have been no voices because of the extraordinary accuracy of the Israeli strikes,” said Martin van Creveld, an Israeli military historian and strategist.

Egypt urged Israel and Hamas to halt the spiralling conflict, but played down hopes of a Cairo-mediated truce. The new military-led government in Cairo is distrustful of Hamas because of the group’s links to Egypt’s Muslim Brotherhood, which was deposed from government last year.

“There is no mediation, in the common sense of the word,” said Badr Abdelatty, an Egyptian foreign ministry spokesman. “Egyptian diplomatic efforts are aimed at immediately stopping Israeli aggression and ending all mutual violence. [Egyptian] contacts have not yet achieved a result.”

As tanks continued to mass on the Gaza border, Benjamin Netanyahu, the Israeli prime minister, said there would no let up in the military operation until Hamas stopped its rockets.

“The operation will expand and continue until the rocket firing on our cities stops, and the quiet returns,” he said. “Hamas will pay a heavy price for firing on Israeli citizens.”

One of the deadliest Israeli strikes took place in the small hours of yesterday, when a missile hit a house in northern Gaza, killing a suspected Islamic Jihad militant and five of his family members.

Further raids to the north and east of Gaza City killed another two women and four children, while a fourth strike on Maghazi refugee camp in central Gaza killed a woman and four of her children. Another five Palestinians died in other raids across Gaza yesterday morning.

Israelis, meanwhile, were sent scurrying into shelters as the Hamas rockets continued, with two reportedly crashing into the sea as far away as the northern port city of Haifa, 100 miles from Gaza. If confirmed, it would be the furthest a rocket fired from Gaza has travelled.

Hamas is thought to have an arsenal of around 10,000 missiles, although so far none has caused any casualties.